1. Field of the Invention
The present invention relates to foamable interpolymer resin particles comprised of polyolefins, e.g. polyethylene, and polymerized vinyl aromatic resin. More particularly, the present invention relates to foamable interpolymer resin particles comprised of a volatile blowing agent ranging from between about 1.5% to about 20 wt % based on the weight of the particles and 0.1 to about 5 parts, preferably 0.1 to 1.0 part, by weight limonene, based on 100 parts by weight of the foamable interpolymer resin particles, as a plasticizer or blowing aid.
2. Background Art
It is known to polymerize a vinyl aromatic monomer, such as styrene, within polyethylene particles. U.S. Pat. No. 3,959,189, issued to Kitamori and assigned to Sekisui Kaseihin Kabushiki Kaisha, discloses a process for producing polyethylene resin particles which comprises suspending in an aqueous medium polyethylene resin particles, adding to the suspension 30 to 100% by weight based on the weight of the particles of a styrene monomer and an initiator for polymerizing the monomer, and polymerizing the monomer inside the polyethylene resin particles. The particles can then be made foamable by impregnating them with a blowing agent, preferably after the polymerization of the styrene monomer and the cross-linking of the polyethylene resin. The blowing agent generally is a volatile blowing agent, i.e. aliphatic hydrocarbons such as n-propane, n-butane, iso-butane, n-pentane, iso-pentane, n-hexane and neopentane; or cycloaliphatic hydrocarbons such as cyclobutane and cyclopentane; and halogenated hydrocarbons such as methyl chloride, ethyl chloride, methylene chloride, trichlorofluoromethane, dichlorofluoromethane, dichlorodifluormethane, chlorodifluoromethane and dichlorotetrafluoroethane, etc. in an amount between 5 to 20% by weight based on the weight of the polyethylene-polystyrene resin particles.
U.S. Pat. No. 4,782,098, issued to Richard B. Allen, et al and assigned to General Electric, Co., discloses foamable interpolymer beads comprising polyphenylene ether resin and a polymerized vinyl aromatic monomer that is polymerized in the presence of a polymerization catalyst to form interpolymerized thermoplastic resin beads. A volatile blowing agent is introduced under pressure into the thermoplastic resin beads.
U.S. Pat. Nos. 4,303,756 and 4,303,757 issued to Kajimura, et al. and assigned to Sekisui Kaseihin Kogyo Kabushiki Kaisha disclose a process for producing expandable thermoplastic resin beads where a vinyl aromatic monomer is polymerized onto the backbone of the polypropylene; and introducing a blowing agent into the thermoplastic resin beads. The blowing agent is the same as that taught in the aforesaid U.S. Pat. No. 4,782,098.
U.S. Pat. No. 4,429,059 issued to Ozutsumi, et al. and assigned to Sekisui Kaseihin Kogyo Kabushiki Kaisha discloses a process for producing foamable polyolefin particles comprising adding a mixture of 3 to 15 parts by weight of a blowing agent and 0.5 to 5 parts by weight of a blowing aid to 100 parts by weight of polyolefin particles. Column 2, lines 63-66, teach that the blowing aid causes the interior of the particles to be in a plastic state to facilitate the foaming of the particles. Representative blowing aids used as a plasticizer are benzene, toluene, xylene, trichlene, perchlene, cyclohexane, carbon tetrachloride, etc.
The particles of the prior art are impregnated with a blowing agent, or in the instance of the aforesaid U.S. Pat. No. 4,429,059 with a blowing agent and a blowing aid. The blowing agents are generally “volatile” hydrocarbons, and the blowing aids are generally considered as hazardous air pollutants (HAPS), as found in the 1990 Federal Clean Air Act Amendments, Section 112. The blowing agents and blowing aids generally are emitted into the environment during the processing and foaming of the particles. Both the producer and the converter of the particles may be forced to reduce the emissions to acceptable regulatory levels, which would involve investing in complicated equipment for collecting the emitted hydrocarbons. The number of foamable particles and foamed articles produced in a given time will be dependent on the permissible regulatory levels for hydrocarbon emissions in a given geographical area. A further disadvantage with the types of blowing aids taught in the aforesaid U.S. Pat. No. 4,429,059, e.g. toluene, benzene, etc. is the associated offensive smell during processing of the foamable particles.
It would be desirable to use a blowing aid that is not considered to be a hazardous air pollutant and that does not have an offensive odor while still providing excellent foamability and molding fusability properties.
It is known to use d-limonene to improve the expandability of primarily styrene-containing expandable polymers. Examples are JP 08208877 (open for public inspection application Aug. 13, 1996) to Sekisui Plastics, Japan; JP 07309968 (granted JP 30609839 on Jul. 10, 2000) to Achilles Corp., Japan; and JP 93202250A to Asahi Chemical Industry Co., Ltd. (Hei5-202250).
The use of limonene in styrene as a plasticizer or as a tackifier is further exemplified in: U.S. Pat. No. 5,222,378; EP 0 701 589 B1 (The Dow Chemical Co.) which is also WO 94/19383 (PCT/US94/06112); and U.S. Pat. No. 4,959,412(Arizona Chemical Company).
JP 11080412 (Achilles Corporation) (Entitled: “Heat-resistant cellular styrene polymer-polyoxyphenylene”) teaches the use of limonene in a blend of styrenic resin and a polyphenylene ether-based resin. The abstract teaches that the particles contain 3-15 parts volatile blowing agent-impregnated polymer particles consisting of 50-90 parts styrene polymers and 10-50 parts poly(phenylene ethers) and 0.1-5 parts (for 100 parts polymer particles) limonene. A composition containing polystyrene, poly(phenylene ether), ethylenebis(stearamide), pentane, and limonene was extruded, pelletized, and molded to give a test piece showing good heat resistance with short molding cycle time. It is stated that the addition of limonene in the base resin can improve the foaming ability of the base resin, and the molding time and cooling time can be shortened.
The prior art does not provide for interpolymer particles comprising a polyolefin, e.g. polyethylene and an in situ polymerized vinyl aromatic resin forming an interpenetrating network of polyolefin and vinyl aromatic resin particles, and which particles contain a blowing aid that is not considered to be a hazardous air pollutant; that is considered as having a pleasant smell; and that is more effective as a blowing aid or plasticizer than those of the prior art.